Modes of interaction of myoglobin with diatomic ligands like oxygen, carbon monoxide, and nitric oxide are probed with ambient and cryogenic temperature photolysis. Oxygen plays fundamental roles in the biological oxidative metabolism required for efficient conservation of energy and drug detoxification. Carbon monoxide and nitric oxide are well-known air pollutants which bind with hemoproteins. Nitric oxide-hemoprotein complexes are frequently observed as intermediates/byproducts of metabolism of nitrogeneous carcinogens and nitrogen fixation. Thus, understanding of the mode of interaction of myoglobin with these diatomic ligands is of vital importance in elucidation of the mechanism of biological oxidation. Sperm whale and bovine myoglobins, their site- directed mutants, and cobalt-substituted derivatives are prepared. Visible and infrared (IR) transient kinetic studies are Carried out for the photolysis of myoglobin-ligand complexes in sub-pico to millisecond timescales in order to analyze geminate and bimolecular recombination processes and to determine geminate and quantum yields of photolysis. The geometry of the bound ligands in myoglobin will be determined by ambient temperature ultrafast/polarized IR or low temperature single crystal electron paramagnetic resonance (BPR) techniques. Electronic and stereochemical structures of primary photolyzed states of myoglobin, cobalt-myoglobin and their site-directed mutants and the liberated ligands will be characterized by low temperature IR, EPR, and resonance Raman spectroscopy. The effects of site-directed mutations on structure and reactivity of myoglobin will be assessed in order to determine how ligand affinity is controlled by stereochemical and electronic properties of the distal structure.